Magnetic isotopes as a means to elucidate Earth and environmental chemistryстатья
Статья опубликована в высокорейтинговом журнале
Информация о цитировании статьи получена из
Web of Science,
Scopus
Статья опубликована в журнале из списка Web of Science и/или Scopus
Дата последнего поиска статьи во внешних источниках: 24 октября 2018 г.
Аннотация:In Earth and environmental chemistry, magnetic isotopes provide a universal means to identify reaction mechanisms. Mass-
independent fractionation of isotopes as a signature of a mechanism occurs by two ways: first, via the magnetic isotope effect
(MIE), which is controlled by magnetic, or hyperfine, coupling between unpaired electrons and magnetic nuclei in
paramagnetic species (particularly, in radicals), and, second, via the nuclear volume effect (NVE), which is induced by the
volume difference between isotopic nuclei. The MIE is the dependence of the reaction rates on the nuclear magnetic moment
of reactants and fractionates magnetic and nonmagnetic isotopes, whereas NVE fractionates isotopes with different nuclear
volumes. Both effects, MIE and NVE, are supposed to coexist in condensed phases. A decisive test for their differentiation is
illustrated by the example of radical pairs with mercury nuclei. Namely, if isotope fractionation is controlled by MIE, the
ratio D201Hg/D199Hg is expected to be in the range of 1.05 ± 1.25 for isotopic enrichment and 0.80 ± 0.92 for depletion. If
isotope fractionation is controlled by NVE, this ratio is estimated to be in the range of 0.50 ± 0.62. In contrast to MIE-induced
bidirectional fractionation controlled by the direction of coherent spin conversion of the radical pair (triplet ± singlet or vice
versa), the NVE induces unidirectional, universal isotope fractionation, which is almost independent of the reaction
mechanism. In contrast to MIE which exhibits inversion of the fractionation sign depending on the spin multiplicity of
reactants, NVE is incompatible with the inversion of the fractionation sign. The MIE is an unambiguous indicator of the
radical mechanisms and dominates in chemical reactions, whereas NVE prevails in nonchemical processes. Chemical
scenarios of MIE-induced oxygen, sulfur, iron, silicon, tin, mercury, germanium and uranium isotope fractionation in
photostimulated and dark reactions are analyzed in terms of reaction mechanisms including reactions in living organisms. In
conclusion, some restrictions, uncertainties and problems in Earth and environmental chemistry are discussed.